The prior art includes filters for removing particulates from various dielectric fluids, such as oils and hydraulic fluids, by employing mechanical filtration. When mechanical filtration is employed to filter very fine particles of, for example, from about 3 to 5 microns, difficulties may be encountered because these relatively small particles require large bulk and volume filters to avoid significant system pressure drops caused by the necessarily small openings in the filter.
Alternatives to mechanical filters for fine particle filtration of dielectric fluids include electrostatic filters where the fluid is made to pass through a number of electrodes which are alternately charged with relatively high positive and negative voltages. Porous filter material is placed between the electrodes for trapping the particulates. It appears that the particulates, when subjected to the electric fields created by the application of voltage to the electrodes, are filtered in one of two possible ways. The filter material itself may be charged with the particulates being attracted to the filter material itself. More likely, however, the particulates are charged, either positively or negatively, depending on their composition, and the oppositely charged particles will be attracted to each other and eventually form a floc of larger particulate clusters which will be large enough to be trapped in the filter material. Whenever enough clusters form to effectively block the filter, or produce an undesirable pressure drop, the filter must be replaced.
While these electrostatic filters have evolved over a number of years, these filters still present a number of problems. Among these problems are the overall costs, difficulties in assembly, and inability to test to determine whether the assembly has been properly accomplished. A very significant problem, moreover, is the debilitating or dysfunctional effect of relatively small amounts of water, or other conductive agents, present in the dielectric fluid. This water or these agents may cause a short circuit between adjacent electrodes. Alternatively, voltage may have to be reduced to an amount below the required voltage for acceptable performance of the electrostatic filter.